The present invention relates to vibration in mechanical systems and, more particularly, relates to an apparatus and associated method for mounting a payload to a vehicle and attenuating vibration therebetween.
Satellites, spacecraft and other payloads are typically launched into orbit using a launch vehicle, such as a rocket. Most payloads are attached to the launch vehicle only at the base of the payload such that the payload is cantilevered to the vehicle. During launch, and while the payload is being transported to its proper orbit and velocity, the payload is subjected to a severe vibration and acoustic environment that creates dynamic loads or vibroacoustic loads (referred to herein as xe2x80x9cvibrationxe2x80x9d) that are transmitted to the payload from the launch vehicle. The vibration can be the result of environmental effects such as wind gusts, as well as events such as liftoff, motor ignitions and shutdowns, fuel depletion and jettisons of fairings, hatches, and booster stages. The vibration can have large amplitudes over a wide frequency range that can damage a payload or lead to on-orbit malfunctions or reduced operating lifetime. Due to the cost associated with the manufacture and launching of most payloads, vibration is considered to be an important factor in the structural design of both payloads and launch vehicles.
Most payloads are mounted on a rigid, inflexible payload support, which transmits all of the vibration directly to the payload. Conventional approaches to improving launch survival of payloads have typically involved stiffening the structural components of the payload, as opposed to providing a means of reducing the loads. However, structural stiffening requires a complete redesign and analysis, as well as the use of exotic and/or expensive materials and can necessitate an undesirable increase in the overall weight of the payload. In addition, the stiffened structural components still must undergo extensive and expensive testing to ensure that the payload will have a reasonable probability of launch survival.
Other approaches to improve launch survival have involved the use of flexible materials to isolate and reduce the transmission of vibration between the vehicle and payload. However, such approaches typically rely on friction to support shear loads, which can negatively affect the flexibility and integrity of the flexible materials.
Thus, there remains a need to replace the conventional design approach of structural stiffening with a vibration isolator that can securely attach a payload to a vehicle while at the same time effectively attenuate the transmission of vibration from the vehicle to the payload. The vibration isolator should be capable of supporting the payload under normal operating conditions, which include positive and negative acceleration in all three axes, attenuating vibration in six axes without relying on friction to support shear loads. The isolator should be capable of energy dissipation using damping or another process. In addition, the isolator must be such that it can be manufactured and assembled with a minimum number of parts to reduce the overall cost of manufacture, reduce assembly time, and minimize weight.
The present invention provides a vibration isolator that can securely attach a payload to a vehicle or a base, as well as effectively isolate the payload from vibration about six axes. Instead of the conventional stiff payload support, the present invention allows relative motion between the support and payload by placing a compliant material in the load path. The vibration isolator includes at least one elastomeric member, which can be formed of silicone, natural and synthetic rubber, or any other elastomer having a relatively high density, modulus of resilience, modulus of elasticity, and material damping. The vibration isolator also includes a first support and a second support spaced from the first support. The first and second supports cooperate with the plurality of elastomeric members to attenuate vibration between the supports.
In one embodiment, at least one, and preferably both of the first and second supports have a circular configuration. The first and second supports can be formed of metallic or composite materials, including, aluminum, AA 2000 series aluminum alloys, AA 6000 series aluminum alloys, AA 7000 series aluminum alloys, titanium, steel, carbon fiber composites, fiberglass fiber composites, or aramid fiber composites. In another embodiment, at least one of the first and second supports is formed of two interlocking members. The vibration isolator also includes at least one fastener that is structured to mount the elastomeric member(s) between the first and second supports such that the elastomeric member(s) allow relative motion and isolate vibration transmitted between the supports. In one embodiment, the stiffness of the vibration isolator is substantially proportional to the bulk modulus of elasticity of the elastomeric member(s).
In one embodiment, at least one elastomeric member is secured between the first and second supports. The securing step can comprise inserting at least one fastener through the first and second supports and the elastomeric member(s) positioned therebetween. The elastomeric member(s) allow relative displacement to thereby damp vibration between the first and second supports. The stiffness of the elastomeric member(s), and thus the entire isolation system, is substantially proportional to the bulk modulus of elasticity of the elastomeric member(s). The modular nature of the elastomeric members allows them to be changed individually or in groups in order to xe2x80x9ctunexe2x80x9d the stiffness and damping properties of the invention depending on the mass of the payload and the desired isolation and damping characteristics.
The first support of the vibration isolator preferably defines a first raised portion and the second support preferably defines a second raised portion. The first and second raised portions are structured to cooperate so as to define a recess therebetween adapted to at least partially receive the elastomeric member(s). In one embodiment, the first support defines a plurality of first raised portions and the second support defines a plurality of second raised portions, each of the first raised portions uniquely corresponding to one of the second raised portions. In another embodiment, the second raised portion of the second support comprises a pair of flanges and a web portion extending therebetween and the first raised portion of the first support comprises a second web portion and a flange. According to this embodiment, the second web portion of the first raised portion extends at least partially between the pair of flanges of the second raised portion and the flange of the first raised portion extends from the second web portion toward the web portion of the second raised portion. In yet another embodiment, the first support of the vibration isolator defines a first raised portion having a generally T-shaped configuration and the second support defines a second raised portion having a generally C-shaped configuration.
In still another embodiment, the present invention provides a mounting system including a payload and a base for supporting the payload. The mounting system includes a vibration isolator, as set forth above, for isolating and damping vibration between the base and the payload. In one embodiment, the first support is attached to the payload and the second support is attached to the base. In another embodiment, the first support is attached to the base and the second support is attached to the payload.
The present invention also provides a method of energy dissipation, which is provided by the elastomeric member(s) positioned between the first and second supports. Relative motion between the first and second supports strains the elastomeric member(s) which internally dissipate energy through self-heating. Any such heating is conducted away from the elastomeric member(s) by heat conduction to the first and second supports.
In another embodiment, the method includes providing a first support defining a first raised portion having a generally T-shaped configuration. In one embodiment the first support is attached to a payload. A second support is provided defining a second raised portion having a generally C-shaped configuration. In one embodiment, the second support is attached to a vehicle or a base. At least one elastomeric member is positioned at least partially between the first and second supports. In one embodiment, the at least one elastomeric member is secured between the first and second supports. The securing step can comprise inserting at least one fastener through the first and second supports and the at least one elastomeric member positioned therebetween. The at least one elastomeric member allows relative motion to thereby isolate and to also damp vibration between the first and second supports.
The present invention positions one or more compliant, elastomeric members between the first support and the second support. The compliant elastomeric member(s) provide isolation and damping properties. However, in cases where loads or accelerations result in a stretching axial load occurring at one or more of the elastomeric members, the fastener securing the elastomer between the supports is placed in tension. For this situation, elastomeric washers are placed between the head of the fastener and the first and second supports to ensure that the reactions are passed through the elastomeric member or members.
Accordingly, there has been provided a vibration isolator that can securely attach a payload to a vehicle while at the same time effectively isolate and damp the transmission of vibration between the base and the payload. The vibration isolator of the present invention is capable of damping the transmission of vibration about all axes. In addition, the isolator is such that it can be manufactured and assembled with a minimum number of parts to reduce the overall weight of the isolator so that the isolator can be easily assembled and disassembled. If desired, the support next to the payload can be incorporated into the payload structure, and the support next to the base can be incorporated into the base structure, for the purposes of reducing weight and height. It is also apparent that the invention can equally be used to support a vibrating payload on a fixed base structure, an application suited for mounting unbalanced or vibratory machinery or other items onto a fixed base without transmitting the vibrations to that base structure.